British police are investigating a stabbing attack at a London Underground station that left three people injured, one seriously, as a terrorist incident.

A spokesman for the Metropolitan Police said officers were called to an incident at Leytonstone station, where a 29-year-old man was reportedly threatening members of the public with a knife.

One man, believed to be aged 56, was seriously injured in the attack and two other people had minor injuries, police said.

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The suspect was arrested and was in police custody, a statement said.

“We are treating this as a terrorist incident. I would urge the public to remain calm, but alert and vigilant,” said Commander Richard Walton, who leads Metropolitan Police Counter Terrorism Command.

An eyewitness said the attacker had screamed about Syria.

“I just saw a lot of people running but I ignored it and kept walking to get my train, but suddenly what I saw I couldn’t believe my eyes and what I saw was a guy with a knife,” he told The Guardian.

“As he was coming out this is what he said: ‘This is what happens when you f*** with mother Syria, all of your blood will be spilled’.”

Footage of the incident posted on social media showed a large pool of blood on the station’s floor.

Shouts are heard as the suspect is seen remonstrating with people before swinging at one of them.

Footage shows an officer shouting “drop the knife” and firing a Taser.

One man at the scene shouted: “You ain’t no Muslim! You’re no Muslim! You ain’t no Muslim!”

VIDEO WARNING: VIOLENCE, COARSE LANGUAGE

https://youtu.be/s7DaWANyw8A

Attacker had 7.5-centimetre knife: eyewitness

An eyewitness told the BBC that one victim was taken out on a vertical stretcher.

Witness Michael Garcia, 24, a financial analyst from Leytonstone, said he had seen “a guy, an adult, lying on the floor with a guy standing next to him brandishing a knife of about 7.5 centimetres,” the BBC reported.

“He was screaming ‘go on, then, run’ to everyone else. He was pacing back and forth next to the guy on the floor.”

Britain is on its second-highest alert level of “severe”, meaning a militant attack is considered highly likely.

This is mainly due to the threat the authorities say is posed by Islamic State (IS) militants in Syria and Iraq and their encouragement of supporters to carry out attacks in their homelands.

British MPs approved the bombing of IS targets in Syria on Wednesday. Britain’s air force has since carried out two bombing raids.

Britain suffered by far its worst militant Islamist attack in July, 2005, when 52 people were killed by suicide bombs on underground trains and a bus.

A beach attack in Tunisia in June this year that killed 30 British holidaymakers was the biggest loss of British lives in such an incident since the July 2005 London bombings.

IS claims that the married couple who killed 14 people in a mass shooting in California, which US authorities are investigating as an act of terrorism, were followers of the militant group.

-with agencies